Sarah Kurtz

Strong Internal and External Luminescence as Solar Cells Approach the Shockley–Queisser Limit

Absorbed sunlight in a solar cell produces electrons and holes. However, at the open-circuit condition, the carriers have no place to go. They build up in density, and ideally, they emit external luminescence that exactly balances the incoming sunlight. Any additional nonradiative recombination impairs the carrier density buildup, limiting the open-circuit voltage. At open circuit, efficient external luminescence is an indicator of low internal optical losses. Thus, efficient external luminescence is, counterintuitively, a necessity for approaching the Shockley–Queisser (SQ) efficiency limit. A great solar cell also needs to be a great light-emitting diode. Owing to the narrow escape cone for light, efficient external emission requires repeated attempts and demands an internal luminescence efficiency 90%.

High-efficiency GaInP∕GaAs∕InGaAs triple-junction solar cells grown inverted with a metamorphic bottom junction

The authors demonstrate a thin, Ge-free III–V semiconductor triple-junction solar cell device structure that achieved 33.8%, 30.6%, and 38.9% efficiencies under the standard 1sun global spectrum, space spectrum, and concentrated direct spectrum at 81suns, respectively. The device consists of 1.8eV Ga0.5In0.5P, 1.4eV GaAs, and 1.0eV In0.3Ga0.7As p-n junctions grown monolithically in an inverted configuration on GaAs substrates by organometallic vapor phase epitaxy. The lattice-mismatched In0.3Ga0.7As junction was grown last on a graded GaxIn1−xP buffer. The substrate was removed after the structure was mounted to a structural “handle.” The current-matched, series-connected junctions produced a total open-circuit voltage over 2.95V at 1sun.

Modeling of two‐junction, series‐connected tandem solar cells using top‐cell thickness as an adjustable parameter

Theoretical efficiencies are calculated for two‐junction, series‐connected solar cells using air mass 1.5 global and direct irradiance spectra. For band‐gap combinations previously limited by a low bottom‐cell current, thinning of the top cell is shown to result in significant increases in the theoretical efficiencies. The increases are primarily due to increased short‐circuit currents, since current matching is achievable. Smaller gains are also seen in the open‐circuit voltages of the thinner cells when a low surface‐recombination velocity is assumed. Thus, a number of material combinations which previously could only be used in four‐terminal configurations can now be considered for series‐connected two‐junction solar cells.

Structural changes during annealing of GaInAsN

The alloy GaInAsN has great potential as a lower-band-gap material lattice matched to GaAs, but there is little understanding of what causes its poor optoelectronic properties and why these improve with annealing. This study provides information about the structural changes that occur when GaInAsN is annealed. The Fourier transform infrared spectra exhibit two primary features: a triplet at ∼470 cm−1 (Ga–N stretch) and two or three bands at ∼3100 cm−1 (N–H stretch). The change in the Ga–N stretch absorption can be explained if the nitrogen environment is converted from NGa4 to NInGa3 after annealing. The N–H stretch is also changed after annealing, implying a second, and unrelated, structural change.

THE INFLUENCE OF SPECTRAL SOLAR IRRADIANCE VARIATIONS ON THE PERFORMANCE OF SELECTED SINGLE-JUNCTION AND MULTIJUNCTION SOLAR CELLS

Abstract The sensitivities of selected single-junction and multijunction cells to variations in solar irradiance are presented. The one-sun spectral irradiance is varied as a function of air mass, optical aerosol depth (turbidity) and amount of precipitable water vapor for direct-normal and global-normal geometries. Several devices, including one-, two- and three-junction devices with low and high bandgaps and either series- or independent-connection schemes, were investigated. The effects of air mass and turbidity on the consistency of high-bandgap device performance are shown to be greater than the effect of precipitable water vapor. Low-bandgap devices are less affected by variations in air mass and turbidity, but are more sensitive to high water-vapor conditions. The efficiency gained by redesigning a multijunction device for the latitude at which it is expected to be used is shown to be less than about 3% (relative).

Projected performance of three- and four-junction devices using GaAs and GaInP

This paper explores the efficiencies expected for three- and four-junction devices for both space and terrestrial applications. For space applications, the effects of temperature and low concentration are investigated. For terrestrial applications, a concentration of 500 suns is assumed and the theoretical efficiencies are calculated as a function of spectral variations including the effects of air mass, turbidity, and water-vapor content.

Terawatt-scale photovoltaics: Trajectories and challenges

Coordinating technology, policy, and business innovations The annual potential of solar energy far exceeds the worlds total energy consumption. However, the vision of photovoltaics (PVs) providing a substantial fraction of global electricity generation and total energy demand is far from being realized. What technical, infrastructure, economic, and policy barriers need to be overcome for PVs to grow to the multiple terawatt (TW) scale? We assess realistic future scenarios and make suggestions for a global agenda to move toward PVs at a multi-TW scale.

The difference between standard and average efficiencies of multijunction compared with single-junction concentrator cells

Abstract The theoretical performances of ideal single- and multijunction cells are compared at 100 × concentration under a range of cloudless-sky conditions. The sensitivities of device performance to cell temperature and spectral variations are shown to depend on the number of junctions (one, two or three), the way in which the junctions are connected (series, parallel or independent), and the band gaps of the devices. The average performances of all of the multijunction devices surpass that of a single-junction GaAs device, but the inconsistency in performance of some of the multijunction devices is significant for large variations in cell temperature and incident spectrum. The choice of band gap and connection scheme is more important than the number of junctions in determining the consistency of device performance.

Outdoor rating conditions for photovoltaic modules and systems

Historically, #at-plate photovoltaic modules have been given a ‘peak-watta rating indicating the power generated under 1000 W/m2 global irradiance at a standard temperature. However, questions have arisen regarding the direct-normal irradiance, ambient or cell temperature, and wind speed (when it is specied) that should be used for evaluating the performance of #at-plate and concentrator modules. By studying the conditions that are observed when the global irradiance on a 2-axis-tracked surface is 1000 W/m2, our analysis provides an objective, quantitative basis for the choice of the ‘peak-watta rating conditions for both types of collectors. These observed conditions are consistent with commonly used values of 850 W/m2 for direct-normal irradiance and 203C for ambient temperature. Evidence is given that wind speed should be increased from the commonly used 1 m/s to a more frequently observed 4 m/s. ( 2000 Elsevier Science B.V. All rights reserved.

Annealing-induced-type conversion of GaInNAs

When grown by metalorganic chemical vapor deposition (MOCVD), nominally undoped GaInNAs is commonly observed to have an acceptor concentration of ∼1017u200acm−3. However, after annealing in the MOCVD reactor at a temperature of 650u200a°C, p-type GaInNAs sometimes converts to n type with an electron concentration of ∼1017u200acm−3. This n-type material has a slightly higher electron mobility (∼400–450 cm2/Vu200as) than has usually been reported for 1 eV GaInNAs. Secondary ion mass spectroscopy shows significant hydrogen and some carbon contamination of these layers. The type conversion is correlated with both the nitrogen and hydrogen concentration and is relatively insensitive to the choice of growth precursors (trimethylgallium versus triethylgallium, or unsymmetric dimethylhydrazine versus nitrogen trifluoride). The data are consistent with theoretical predictions that the donors arise from a N–H complex. Annealing in the absence of hydrogen reduces the background acceptor concentration, but does not produce an electron concentration as high as 1017u200acm−3.When grown by metalorganic chemical vapor deposition (MOCVD), nominally undoped GaInNAs is commonly observed to have an acceptor concentration of ∼1017u200acm−3. However, after annealing in the MOCVD reactor at a temperature of 650u200a°C, p-type GaInNAs sometimes converts to n type with an electron concentration of ∼1017u200acm−3. This n-type material has a slightly higher electron mobility (∼400–450 cm2/Vu200as) than has usually been reported for 1 eV GaInNAs. Secondary ion mass spectroscopy shows significant hydrogen and some carbon contamination of these layers. The type conversion is correlated with both the nitrogen and hydrogen concentration and is relatively insensitive to the choice of growth precursors (trimethylgallium versus triethylgallium, or unsymmetric dimethylhydrazine versus nitrogen trifluoride). The data are consistent with theoretical predictions that the donors arise from a N–H complex. Annealing in the absence of hydrogen reduces the background acceptor concentration, but does not produce an electr...